Physical Properties of High-Temperature Superconductors

By Rainer Wesche

A much-needed update on complex high-temperature superconductors, focusing on materials aspects; this timely book coincides with a recent major break-through of the discovery of iron-based superconductors. 

It provides an overview of materials aspects of high-temperature superconductors, combining introductory aspects, description of new physics, material aspects, and a description of the material properties   This title is suitable for researchers in materials science, physics and engineering. Also for technicians interested in the applications of superconductors, e.g. as biomagnets

• Language: English
• Publisher: Wiley
• Release date: May 13, 2015
• ISBN: 9781118696675

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About the Author

Rainer Wesche studied physics at the University of Constance, Germany (M.Sc. in 1984) and received his Ph.D. in physics from the University of Constance, Germany, in 1988. From 1989 to 1993, he was a research scientist at the Paul Scherrer Institute, Switzerland, where he led an experimental study of high-current applications of high-Tc superconductivity funded by the Swiss National Science Foundation. Since 1994, he has been a research scientist at the Swiss Federal Institute of Technology Lausanne (Centre de Recherches en Physique des Plasmas (CRPP)). His present research is in the field of applied superconductivity.

Series Preface

Wiley Series in Materials for Electronic and Optoelectronic Applications

This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much-needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers, and technologists, engaged in research, development, and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices, and circuits for the electronic, optoelectronic, and communications industries.

The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics. The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure-property relationships have always been fundamentally and intrinsically important to materials science and engineering.

Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials, and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

Arthur Willoughby

Peter Capper

Safa Kasap

Preface

The now more than 100-year-long history of superconductivity started with the discovery of the phenomenon by H. Kammerlingh Onnes in 1911 at the University of Leiden, The Netherlands. The discovery of J.G. Bednorz and K.A. Müller in 1986 that the superconducting state can exist in complex oxides above 30 K revitalized the field of superconductivity. This breakthrough started a race to find cuprate high-Tc superconductors with higher and higher critical temperatures. As early as the following year, YBa2Cu3O7−x, the first superconductor with a critical temperature above 77 K, the boiling point of liquid nitrogen, was discovered. Today a large number of cuprate high-Tc superconductors with critical temperatures well above this are known.

The discovery of J.G. Bednorz and K.A. Müller opened up the search for the pairing mechanism in the cuprate high-Tc superconductors. Furthermore, the investigation of the phase diagrams and the processing conditions of complex cuprate superconductors led to progress in the materials science of complex multicomponent compounds. Last but not least, the potential for reduced operation costs resulting from the use of liquid nitrogen as coolant renewed interest in power applications of superconductivity.

This book provides an overview of the known cuprate- and iron-based high-Tc superconductors and their physical properties. In addition, the special case of the intermediate-temperature superconductor MgB2 is considered. Further aspects presented are the synthesis of these materials, the manufacture of superconducting wires and tapes, and the deposition of superconducting films. The book should be suitable for use in graduate-level courses on superconductivity. A large number of figures, tables, and references illustrate the status of research and development in the field in mid-2014.

In Chapter 1, the milestones in the history of superconductivity are briefly described. A special aspect of importance is the development of the maximum known critical temperatures in metals, oxides, molecular, and iron-based superconductors. The fundamental physical principles of normal-state electrical conductivity and the well-known characteristics of metallic superconducting elements (Type I superconductors) are presented in Chapter 2. Because the superconducting state in these materials can be destroyed by magnetic fields as small as 100mT, they are not suitable for magnet applications. The main results of the Bardeen–Cooper–Schrieffer (BCS) theory, the microscopic quantum theory of superconductivity in conventional metallic superconductors, are briefly described in Chapter 3. In addition, it is shown that superconductivity is a macroscopic quantum phenomenon, which is reflected in flux quantization and tunneling effects. In Chapter 4, the properties of Type II superconductors are presented. Because normal and superconducting regions can coexist in Type II metallic superconductors, the magnetic fields required to destroy superconductivity can exceed 20T. In contrast, the high-Tc materials are extreme Type II superconductors with upper critical fields of the order of 100 T.

An overview of the most important families of cuprate high-Tc superconductors is given in Chapter 5. The crystal structures of cuprate superconductors are described in Chapter 6. Empirical rules for the critical temperature of cuprate high-Tc superconductors are discussed in Chapter 7. The generic phase diagram of hole-doped cuprate superconductors is presented in Chapter 8. Aspects to be discussed are how high-Tc superconductivity can come into existence in an insulating antiferromagnetic parent compound, in which mobile holes are doped into the CuO2 planes, the superconducting order parameter has d-wave symmetry, and a pseudogap exists. In Chapters 9–13, the physical properties of the cuprate high-Tc superconductors are described. Chapter 14 focuses on the synthesis of cuprate superconductor powders and the manufacture of bulk material. In Chapter 15, the manufacture and the performance of first- (Ag/Bi-2212 round wires and tapes, Ag/Bi-2223 tapes) and second-generation (biaxially textured RE-123 (where RE is yttrium or another rare earth element) coated conductors) cuprate high-Tc superconductor wires and tapes are discussed. Chapter 16 is devoted to cuprate high-Tc superconductor films deposited on single-crystal substrates, and to the achieved critical temperatures and transport critical current densities.

Chapter 17 provides an overview of the physical properties of MgB2, the status of the development of MgB2 wires and tapes, and the preparation of MgB2 films. An overview of the recently discovered iron-based superconductors and their properties is provided in Chapter 18.

Finally, an outlook on future research and development is given in Chapter 19. Future research is expected to focus in two directions, namely the pairing mechanisms in cuprate and iron-based superconductors, and the development of high-Tc superconductors and MgB2for magnet and power applications.

Rainer Wesche

Acknowledgment

I wish to thank P. Bruzzone, the head of the superconductivity section of CRPP, for his support and encouragement. The careful reviewing and many constructive suggestions on the manuscript by J.F. Crawford were especially appreciated. The kind permission from Springer Science+Business Media B.V. to reprint parts of my previous monograph High-Temperature Superconductors: Materials, Properties, and Applications (ISBN 0-7923-8386-9) published by Kluwer Academic Publishers in 1998 is gratefully acknowledged.

List of Tables

Nomenclature

Chapter 1

Brief History of Superconductivity

1.1 Introduction

In 1911, the phenomenon of superconductivity was discovered by Heike Kammerlingh Onnes at the University of Leiden (The Netherlands). The centenary of this discovery was celebrated with a joint conference of the European Applied Superconductivity Conference (EUCAS 2011), the International Superconductive Electronics Conference (ISEC 2011), and the International Cryogenic Materials Conference (ICMC 2011) on September 2011 in The Hague (The Netherlands) [1]. Because of this anniversary, it seems to be worthwhile to briefly describe the history of superconductivity. This overview of the most important events in the history of superconductivity allows us to consider the high-temperature superconductors (HTSs) and their properties in the broader context of superconductivity in general. The most important milestones in the field of superconductivity are listed in Table 1.1.

Table 1.1 Important milestones in the history of superconductivity

1.2 Milestones in the Field of Superconductivity

1.2.1 Early Discoveries

In 1908, the successful development of helium liquefaction techniques in the laboratory of H. Kammerlingh Onnes at the University of Leiden made temperatures accessible down to about 1 K [2]. One of the first aspects to be investigated was the electrical resistance of pure metals at very low temperatures. At that time, no proper theory of the electrical resistivity of metals existed. It was already known that the electrical resistance of metals decreases with decreasing temperatures. At the lowest temperatures, a nearly temperature-independent residual resistivity was observed for platinum and gold. This residual resistivity was found to decrease with increasing purity of the investigated sample. Mercury was selected for the further investigations because this low melting-point metal could be purified by repeated distillation. In 1911, Kammerlingh Onnes found that, at a temperature of approximately 4.2 K, the electrical resistivity of mercury suddenly drops to a value too small to be measured. The temperature at which the superconducting state is reached is called the transition or critical temperature c01-math-0001 . The resistance versus temperature data measured by Kammerlingh Onnes are shown in Figure 1.1. The loss of resistivity occurred within a temperature interval of 0.04 K. The unexpected phenomenon of superconductivity had been discovered [2]. Soon after this discovery, it was found in Leiden that tin c01-math-0002 and lead c01-math-0003 are also superconducting metals [2].

c01f001

Figure 1.1 Resistance vs. temperature plot obtained for mercury by Kammerlingh Onnes

(adapted from [2])

The most remarkable property of the superconducting state is field exclusion. In 1933, Meissner and Ochsenfeld observed that the magnetic field is expelled from the interior of a superconductor, which is cooled below the transition temperature in the presence of a small magnetic field [3]. This effect, nowadays known as the Meissner effect, cannot be explained by zero resistance and is in fact a second characteristic property of the superconducting state.

In 1935, the brothers Fritz London and Heinz London developed the first phenomenological theory of superconductivity [4]. The London equations (see Section 2.4) provide a theoretical description of the electrodynamics of superconductors, including the Meissner effect. In a thin surface layer, just inside the superconductor, screening currents flow without resistance, which cancel the applied magnetic field in the interior of the superconductor. The thickness of this layer, known as the London penetration depth, is a characteristic of the superconductor in question. In addition, London recognized that superconductivity is an example of a macroscopic quantum phenomenon. The behavior of a superconductor is governed by the laws of quantum mechanics like that of a single atom, but on a macroscopic scale [26, 27].

In 1941, Aschermann et al. found superconductivity in NbN with a transition temperature of 15 K [5]. Most superconductors discovered before 1941 were elemental metals or alloys. An interesting aspect is the relatively high critical temperature in a compound containing the nonmetallic element nitrogen.

1.2.2 Progress in the Understanding of Superconductivity

In 1950, Ginzburg and Landau developed a phenomenological theory of superconductivity [6], based on the principles of thermodynamics and empirical in nature. Abrikosov [28] solved the Ginzburg–Landau (GL) equations and found solutions explaining the penetration of magnetic flux into Type II superconductors (see Chapter 4), leading to the formation of a flux line lattice. The London theory of superconductivity is a special case of the GL theory [29].

In 1957, Bardeen, Cooper, and Schrieffer [8, 9] developed a microscopic quantum theory of superconductivity (BCS theory, see Section 3.2). The electron–phonon interaction leads to a weak attraction between two electrons. This leads to the formation of Cooper pairs consisting of two electrons of opposite spin and momentum. The total spin of a Cooper pair is therefore zero and as the consequence the paired electrons do no longer obey Pauli's exclusion principle. All the Cooper pairs condense into a single ground